In the first light of dawn Martian and Earthman studied the diagram together, but it was already hot when Nick turned his back on the original and reproduced it in another patch of sand. Klev checked it and nodded approval. "Let's go," Nick said, starting to rise. Klev restrained him and fumbled in the torn folds of his clothing to produce a glowing sphere the size of a marble. It seemed to be a portable form of the glow-plates with which he was familiar. "Thanks," he said. "Take it easy until we get back--I hope." The Martian understood the sense though not the actual words. Then he and Susan dashed across the small expanse of sun-baked rock and squeezed into the passage. For a while they followed the tunnel through which they had been ejected, but shortly encountered a branching passage they recognized from Klev's map. It spiraled down, narrower and steeper than the main tunnel, and soon it too branched. Once more they took the steeper route. The air grew stagnant but cool. Most of the way they were forced to stoop or crawl, and three times they encountered sections so constricted that they had to stretch out flat and inch tortuously along. Little by little the air grew humid. At last they came to an almost level passage where they could walk erect, rounded a turn, and their greenish light no longer was reflected by enclosing walls. "Nick," Sue whispered. Her voice reverberated hollowly, dying away in distant echoes that seemed incredibly loud. Nick paused with hand on knife to see if they had been overheard. Finally they tiptoed out along a sloping shelf of rock, out into the great cavern. And then the light shielded in Nick's hand gleamed on a sheet of black, still water. At once both were on their knees, scooping up the precious liquid in their hands, drinking their fill. Satisfied, Nick placed his lips to her ear. "Where are the _voras_?" he breathed. She pointed to the water. * * * * * Together they piled their clothing on the ledge, hooding the light with Nick's shirt so that only a faint glimmer showed. Then they waded out into the chill, unfathomable blackness of the underground lake, holding hands to steady each other. Nick's scalp was tingling and with every step he half expected his bare feet to encounter something soft and _alive_. The water deepened rapidly and soon they had to swim. Nick had never had the opportunity to become an expert, so although he tried to be quiet his arms made small splashing sounds as he raised and lowered them, sounds that echoed sibilantly throughout the gigantic chamber. Sue swam close, invisible in the darkness, and he flinched momentarily as a current of water from her hand felt like something living brushing his side. She grasped his hands, pulling his arms through the motions of a breast stroke that would not require lifting them above the surface. He touched her to show he understood.